On Sun, Jul 19, 2009 at 04:29:33PM +0200, Aurelien Jarno wrote:
> On Wed, Jul 15, 2009 at 12:48:02PM -0700, Richard Henderson wrote:
> > On 07/13/2009 03:16 PM, Matt Turner forwarded:
> >>>>> The problem is that the memchr() function on alpha uses prefetch, which
> >>>>> can cause a page boundary to be crossed, while the standards (POSIX and
> >>>>> C99) says it should stop when a match is found.
> >
> > That's not supposed to matter -- faults from prefetch are supposed to be
> > ignored; see do_page_fault:
>
> The problem is that the "prefech" is not done with $31, but using $1 and
> $3. It is called "prefetch" in the code, but it is more like "read a value
> in advance".
>
Knowing that $31 could be used for prefetch, I have modified the
assembly code from memchr.S to use it. It passes all the testsuite.
Comments are welcome. Then I'll do the alphaev6 version.
diff --git a/sysdeps/alpha/memchr.S b/sysdeps/alpha/memchr.S
index 5d713d5..87c7fb1 100644
--- a/sysdeps/alpha/memchr.S
+++ b/sysdeps/alpha/memchr.S
@@ -119,7 +119,7 @@ $first_quad:
# At least one byte left to process.
- ldq t0, 8(v0) # e0 :
+ ldq zero, 8(v0) # e0 : prefetch next quad
subq t4, 1, a2 # .. e1 :
addq v0, 8, v0 #-e0 :
@@ -138,19 +138,19 @@ $first_quad:
# At least three quads remain to be accessed
- mov t0, t3 # e0 : move prefetched value to correct reg
-
.align 4
$unrolled_loop:
- ldq t0, 8(v0) #-e0 : prefetch t0
- xor a1, t3, t1 # .. e1 :
- cmpbge zero, t1, t1 # e0 :
- bne t1, $found_it # .. e1 :
+ ldq t0, 0(v0) # e0 : load quad
+ xor a1, t0, t1 # .. e1 :
+ ldq zero, 8(v0) # e0 : prefetch next quad
+ cmpbge zero, t1, t1 # .. e1:
+ bne t1, $found_it # e0 :
- addq v0, 8, v0 #-e0 :
+ addq v0, 8, v0 # e1 :
$odd_quad_count:
+ ldq t0, 0(v0) # e0 : load quad
xor a1, t0, t1 # .. e1 :
- ldq t3, 8(v0) # e0 : prefetch t3
+ ldq zero, 8(v0) # e0 : prefetch next quad
cmpbge zero, t1, t1 # .. e1 :
addq v0, 8, t5 #-e0 :
bne t1, $found_it # .. e1 :
@@ -159,8 +159,8 @@ $odd_quad_count:
addq v0, 8, v0 # .. e1 :
bne t5, $unrolled_loop #-e1 :
- mov t3, t0 # e0 : move prefetched value into t0
-$final: subq t4, v0, a2 # .. e1 : a2 <- number of bytes left to
do
+$final: ldq t0, 0(v0) # e0 : load last quad
+ subq t4, v0, a2 # .. e1 : a2 <- number of bytes left to do
bne a2, $last_quad # e1 :
$not_found:
--
Aurelien Jarno GPG: 1024D/F1BCDB73
aurel...@aurel32.net http://www.aurel32.net
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